Magnetoinductive flowmeters and methods for operating magnetoinductive flowmeters of the type referred to above have been well known for some time and are used in a wide variety of applications. The underlying precept of a magnetoinductive flowmeter for fluid media goes all the way back to Faraday who in 1832 proposed applying the principle of electrodynamic induction in flow-rate measurements. According to Faraday's law of induction, a flowing medium that contains charge carriers and passes through a magnetic field generates an electric field intensity perpendicular to the direction of flow and to the magnetic field. A magnetoinductive flowmeter takes advantage of Faraday's law of induction in that a magnet, usually consisting of two magnetic poles, each with a field coil, generates a magnetic field in the measuring tube typically perpendicular to the direction of flow. Within that magnetic field, each volume element of the flowing medium traveling through the magnetic field and containing a certain number of charge carriers contributes its field intensity to a measuring voltage that can be collected through measuring electrodes.
In conventional magnetoinductive flowmeters the measuring electrodes are designed for direct-conductive or capacitive coupling with the flowing medium. This invention addresses flowmeters designed for direct-conductive coupling with the flowing medium.
A salient feature of magnetoinductive flowmeters is the proportionality between the measuring voltage and the flow rate of the medium averaged across the diameter of the measuring tube, i.e. between the measuring voltage and the volumetric flow.
Actual flow-measuring operations employing a magnetoinductive flow-measuring process usually involve periodic alternation of the magnetic field. Prior art has developed a variety of approaches to that effect, such as the use of an alternating field especially by connecting the field coils of the magnet directly to an AC line source which produces a sinusoidal 50 Hz alternating field. Nowadays, however, the general practice is to work with a switched continuous field to avoid transformational interference voltages and line noise potentials. A switched continuous field is produced by feeding the field coils of the magnet a current with a periodic square-wave pattern of periodically alternating polarity. It is also possible, however, to obtain magnetoinductive flow measurements using a pulsating continuous field that is produced by periodically feeding the field coils of the magnet a time-based square-wave current of always the same polarity. The preferred method, however, involves the periodic polarity reversal of the field current since changing the polarity of the magnetic field makes it possible to suppress interference signals such as electrochemical noise.
The voltage between the measuring electrodes, being proportional to the flow rate, is usually quite low, typically in the microvolt range. Measuring that voltage requires high resolution (about 100 nV); in conventional magnetoinductive flowmeters employing the switched constant-field principle, the measuring frequency is in the 1 to 100 Hz range.
The only limiting factor in flow measurements using a magnetoinductive flowmeter essentially consists in adequate electric conductivity of the medium, but in many cases the conductivity of the medium is unknown. A magnetoinductive flowmeter which in simple fashion could determine the conductivity of the medium flowing through the measuring tube would, therefore, offer an added benefit to the user of the magnetoinductive flowmeter. Another added benefit to the user of a magnetoinductive flowmeter would be the ability to determine the leakage rate of an electrode, meaning the condition in which liquid accumulates behind an electrode, typically the reference electrode on the bottom of the measuring tube.